Abstract
The left ventral occipitotemporal cortex (vOT) is considered the key area of the visuo-orthographic system. However, some studies reported that the area is also involved in speech processing tasks, especially those that require activation of orthographic knowledge. These findings suggest the existence of a top-down activation mechanism allowing such cross-modal activation. Yet, little is known about the involvement of the vOT in more natural speech processing situations like spoken sentence processing. Here, we addressed this issue in a functional Magnetic Resonance Imaging (fMRI) study while manipulating the impacts of two factors, i.e., task demands (semantic vs. low-level perceptual task) and the quality of speech signals (sentences presented against clear vs. noisy background). Analyses were performed at the levels of whole brain and region-of-interest (ROI) focusing on the vOT voxels individually identified through a reading task. Whole brain analysis showed that processing spoken sentences induced activity in a large network including the regions typically involved in phonological, articulatory, semantic and orthographic processing. ROI analysis further specified that a significant part of the vOT voxels that responded to written words also responded to spoken sentences, thus, suggesting that the same area within the left occipitotemporal pathway contributes to both reading and speech processing. Interestingly, both analyses provided converging evidence that vOT responses to speech were sensitive to both task demands and quality of speech signals: Compared to the low-level perceptual task, activity of the area increased when efforts on comprehension were required. The impact of background noise depended on task demands. It led to a decrease of vOT activity in the semantic task but not in the low-level perceptual task. Our results provide new insights into the function of this key area of the reading network, notably by showing that its speech-induced top-down activation also generalizes to ecological speech processing situations.
Highlights
Studies of the neural basis of language processing have revealed the involvement of a wide range of brain regions distributed over the temporal, frontal and occipital lobes (Price, 2012)
Inspired by studies that reported the involvement of the left ventral occipitotemporal cortex (vOT) in spoken word processing tasks (Yoncheva et al, 2010; Ludersdorfer et al, 2013, 2015, 2016), the current study further examined whether this key area of the reading network is recruited during a more natural situation like spoken sentence processing
We tested whether vOT responses to spoken sentences depend on two factors that contribute to task difficulty, i.e., task demands and quality of spoken input
Summary
Studies of the neural basis of language processing have revealed the involvement of a wide range of brain regions distributed over the temporal, frontal and occipital lobes (Price, 2012) These widely distributed networks reflect the contribution of both modality-specific sensory-motor and shared higher-level cognitive systems during spoken and written language processing. On the other hand, recruits a hierarchically organized occipitotemporal pathway in the left hemisphere, allowing access to orthographic information form visual input (Dehaene et al, 2005; Vinckier et al, 2007). Several brain-imaging studies have shown that this area can be activated by speech, in the absence of any visual sensory input This is especially the case when the tasks require explicit access to orthographic representations. Such cross-modal activations have mainly been explained by a task-induced top-down activation of orthographic representations stored in the visuo-orthographic processing pathway (Ludersdorfer et al, 2015, 2016)
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